In the Justice challenge, there are two locations that can be farmed and are critical to progressing the story: Light Forest and Deep Woods. Both provide a Pendulum challenge and the latter can be farmed as long as you like (although, you may fail the challenge if you have no defenses set up at the Fort while you farm).

Every time you attempt a Pendulum Gambit on one of these locations, take a step in any direction and then right back onto it in order to re-attempt.

Gear that slows 'Precision' for overall ease and the companion Estrella Fiore for an extra re-attempt may also make your grind easier.